gnu: emacs-magit: Drop the libgit backend.

The libgit backend currently provides next to no features, but
demands extra maintenance overhead.  It is in fact not a usable
backend; thus let's not use it.

* gnu/packages/emacs-xyz.scm (emacs-magit)[arguments]
<#:emacs>: Drop argument.
<#:exclude>: Add sources related to emacs-libgit.
[inputs]: Remove emacs-libgit.

Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

gnu/packages/emacs-xyz.scm

@@ -892,9 +892,11 @@ (define-public emacs-magit
     (build-system emacs-build-system)
     (arguments
      (list
-      #:emacs emacs-no-x             ;module support is required
       #:tests? #t
       #:test-command #~(list "make" "test")
+      #:exclude #~(cons* "magit-libgit.el"
+                         "magit-libgit-pkg.el"
+                         %default-exclude)
       #:phases
       #~(modify-phases %standard-phases
           (add-after 'unpack 'build-info-manual
@@ -942,7 +944,7 @@ (define-public emacs-magit
     (inputs
      (list git perl))
     (propagated-inputs
-     (list emacs-dash emacs-libgit emacs-transient emacs-with-editor))
+     (list emacs-dash emacs-transient emacs-with-editor))
     (home-page "https://magit.vc/")
     (synopsis "Emacs interface for the Git version control system")
     (description